Recently, there is a trend toward personal computers having greater display screens. A user of a personal computer normally have a plurality of rectangular individual screens called windows displayed on the display screen, and the user can efficiently carry out his or her tasks using the plurality of windows kept open on the screen. In order to organize pieces of data stored in a personal computer, a user may create a collection of files, which is called a folder, to manage files of the same type collectively.
As a result of increase in the capacity of storage devices, it has become possible to store an enormous amount of data in a personal computer. Under such circumstances, a user of a computer is sometimes unable to remember what kinds of files have been stored and in which folders. Then, a user may use a file browsing function such as Explorer of Windows (registered trademark) or Finder of MaCintosh to check the content of folders. Alternatively, a tree structure may be displayed to allow a user to check data stored in a personal computer as disclosed in JP-A-2007-26210 (Patent Document 1).